The invention relates to electric machines, in particular electronically commutated electric machines, in which a zero crossing of a winding phase current is detected for actuation, in particular for determination of the rotor position.
Electronically commutated electric machines, such as synchronous motors, asynchronous motors and the like, for example, are known from the prior art. Such an electric machine has a passive rotor, which can be provided with permanent magnets, for example, in order to form rotor poles. A drive force is exerted on the rotor by an external magnetic field being produced. The external magnetic field needs to be generated depending on the present rotor position, with the result that knowledge of the instantaneous rotor position of the rotor is necessary for actuation of such an electronically commutated electric machine.
The external magnetic field is produced with the aid of a stator winding, generally with a plurality of phases. The rotor position is used to determine the phase voltages to be applied to the stator winding and to apply these phase voltages to the winding phases of the stator winding.
The electric machine is actuated with the aid of a driver circuit, which is often formed with the aid of inverter circuits, in order to actuate the winding phases of the electric machine with the aid of H bridge or B6 bridge circuits. The actuation takes place in accordance with an energization pattern which is dependent on the instantaneous rotor position and is selected such that a stator magnetic field produced by the phase voltage is provided so as to lead the exciting magnetic field of the permanent magnets in the rotor in the direction of rotation in order to provide a drive torque.
Methods for detecting the rotor position are sufficiently well known from the prior art. Firstly, the present rotor position can be detected with the aid of a position detector, which gives an indication of the instantaneous rotor position on the basis of an analog or digital measurement signal of a corresponding actuation unit for the electric machine. Such position detectors can detect the instantaneous rotor position with the aid of a plurality of Hall sensors or GMR (Giant Magnetic Resistance) sensors. For this purpose, said sensors can be arranged close to the rotor poles of the rotor or on a magnetic sensor wheel and can indicate the position of the rotor on the basis of the intensity and direction of the magnetic field measured by the position detectors.
Secondly, the rotor position of the rotor of the electric machine can also be detected in accordance with a so-called back-EMF method by evaluation of a profile of an induced voltage in the deenergized state of a relevant winding phase of the stator winding. For this purpose, first the zero crossing of a winding phase current in the relevant winding phase is detected or predicted and the voltage supply for the phase voltage on the winding phase is deactivated for a predetermined measurement time window, which includes the zero crossing. During the measurement time window, the profile of the induced voltage is determined and, on the basis of the profile of the induced voltage, the zero crossing time of a zero crossing of the induced voltage is calculated. The zero crossing time of the induced voltage can be an indication of the instantaneous rotor position.
Until now, the detection of the zero crossing of the winding phase current in one or a plurality of the winding phases is established by measurement of a voltage drop across a shunt or a voltage drop across one of the power switches in the driver circuit. However, these methods are complex and require the provision of additional measures in order to monitor the current profile of a winding phase current in order to be able to establish the zero crossing thereof. In addition, when using shunts, said shunts need to be connected in series with the winding phases, with losses occurring which can impair the efficiency of the motor system.
It is therefore an object of the present invention to determine a method and an apparatus for determining a zero crossing of a winding phase current in a simple manner, with in particular the complexity in terms of design measures being reduced.